One goal in designing digital television receivers is to design a “universal” receiver—one that can receive as many different broadcast standards as possible. This goal may be possible because many television broadcast standards share common features. One design goal for a mobile television receiver is to implement a simple, low-cost design. For instance, this goal may be achieved by simplifying the tuner design and the signal extraction design, and reducing the power processing (to save as much power as possible).
For example, in the ISDB-T (Integrated Services Digital Broadcast-Terrestrial) standard, a broadcast signal includes 13 segments. There are three possible reception modes: all 13 segments, three segments, or one segment. There are multiple ways to construct a receiver, which can include a single tuner or multiple tuners. For example, a receiver may include only a one segment tuner or a 13 segment tuner, which would require adjustments on the transmitter side to accommodate the one tuner at the receiver. A multiple segment tuner (i.e., capable of receiving 13 segments, three segments, or one segment) may be used, but this type of tuner is more complex in operation and construction and is more expensive than other tuner types. A multiple segment tuner may not be as accurate for the one segment case as it is for the 13 segment case, for example. This results from a synchronization problem, especially for a coarse frequency offset (CFO). Synchronization is important to properly receive and display the broadcast signal.
CFO is used to estimate the frequency used. Determining the mode of the signal includes detecting the number of frequencies used per segment in the time domain. A “guard” is provided at the beginning of a transmission frame, to protect against signal leakage between consecutive symbols. In one implementation, the guard is a buffer that includes some repeated information.
The synchronization performance can be limited by the tuner. For example, in ISDB-T, some low-cost tuners exhibit envelope compression. For synchronization, the typical method uses a pilot signal. But this approach has a performance limitation if the receiver is not sufficiently accurate. Using a more accurate tuner can also result in a more complex tuner, in terms of operation and construction.
Generally, in an orthogonal frequency division multiplexing (OFDM) system (such as ISDB-T), the mode and the guard need to be determined prior to performing a fast Fourier transform (FFT) on the received signal. The signal includes a data portion and a cyclic prefix portion. To synchronize the system to obtain the data portion, the FFT window needs to be determined. By determining the mode and the guard of the signal, the FFT window can be determined. These two values identify how large the FFT window is and how large the prefix portion is. With an ISDB-T signal, if only one of the 13 segments of the signal is desired, then the number of prefix and data samples in one symbol may be very small, thereby making detecting the mode and the guard difficult.